System and method for receiving data across an isolation barrier

ABSTRACT

In one embodiment, A system for communication has a receiver for receiving data from a passive transmitter capacitively coupled to the receiver. The receiver has a sensing element having a plurality of terminals configured to be capacitively coupled to the passive transmitter and DC isolated from the passive transmitter.

TECHNICAL FIELD

This invention relates generally to electrical components, and moreparticularly to a system and method for receiving data across anisolation barrier.

BACKGROUND

In a number of electronic applications, for example, power supplies andsafety systems, it is necessary to exchange data between circuits thatare electrically isolated from each other. For example, in an air-bagdeployment controller, two control domains work in parallel to deliver avalidation signal to fire the air-bag. To minimize fault couplingbetween these control domains, each domain has independent clock sourcesand power supplies. Furthermore, portions of an airbag controller areisolated from other portions of a car's electronic system in order toensure that the airbag can be deployed even in the event of a failure ofthe car's electronic system. In some systems, communication takes placeacross an isolation barrier such as an air gap, printed circuit board(PCB), or other isolating material that does not pass direct current(DC). In some systems, communication takes place between circuits thatare not referenced to the same ground potential.

Conventional systems for data exchange over isolation barriers include,for example, optocouplers, capacitive couplers, and/or inductivecouplers, which transmit energy across the isolation barrier. An LED ofan optocoupler transmits optical power to drive an active data level,while inductive and capacitive couplers transmit repetitive currentand/or voltage pulses to be detected by an isolated receiver. Atransmitter for an inductive coupler, for example, generally sends databy passing current pulses though a coil and or a transformer. Acapacitive coupler based on a charge pump principal, on the other hand,sends current pulses by transferring charge from at least one capacitivestorage element at the transmitting side to an isolated receivingelement at the receiving side. Because isolation barriers generally donot pass DC current, even the transmission of static data usuallyrequires continual transmission energy in the form of refresh pulses inorder to prevent data failures.

What is needed are reliable and power efficient systems and methods fortransferring data across an isolation barrier.

SUMMARY OF THE INVENTION

In one embodiment, a system for communication has a receiver forreceiving data from a passive transmitter capacitively coupled to thereceiver. The receiver has a capacitive sensor having a plurality ofterminals configured to be capacitively coupled to the passivetransmitter and DC isolated from the passive transmitter.

The foregoing has outlined, rather broadly, features of the presentinvention. Additional features of the invention will be described,hereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 a-1 d illustrate an embodiment isolation barrier transmissionsystem;

FIGS. 2 a-2 d illustrate embodiment implementations of isolation barriertransmission system;

FIGS. 3 a-3 b illustrate an embodiment isolation barrier transmissionsystem on an integrated circuit substrate;

FIG. 4 illustrates a block diagram of an embodiment synchronousisolation barrier communication system;

FIG. 5 illustrates an embodiment receiver circuit;

FIG. 6 illustrates another embodiment receiver circuit;

FIG. 7 illustrates an embodiment system using two coreless transformer(CT) coils; and

FIG. 8 illustrates an embodiment system using a CT coil in an H-bridgeconfiguration.

Corresponding numerals and symbols in different figures generally referto corresponding parts unless otherwise indicated. The figures are drawnto clearly illustrate the relevant aspects of embodiments of the presentinvention and are not necessarily drawn to scale. To more clearlyillustrate certain embodiments, a letter indicating variations of thesame structure, material, or process step may follow a figure number.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of embodiments are discussed in detail below. Itshould be appreciated, however, that the present invention provides manyapplicable inventive concepts that may be embodied in a wide variety ofspecific contexts. The specific embodiments discussed are merelyillustrative of specific ways to make and use the invention, and do notlimit the scope of the invention.

The present invention will be described with respect to embodiments in aspecific context, namely systems and methods for receiving data acrossan isolation barrier. Embodiments of this invention may also be appliedto systems and methods directed toward other data communication.

In embodiments of the present invention, data is sent across anisolation barrier by a passive transmitter, such as a switch, coupled toa first side of the isolation barrier. A sensing element coupled to asecond side of the isolation barrier receives the data sent by thepassive transmitter by detecting a change in impedance seen at thesecond side of the isolation barrier. As the switch is opened andclosed, the impedance seen by the sensing element changes. Data,therefore, can be sent across the isolation barrier with little or notransmitted power by modulating an impedance. Power consumption on thereceiver side of embodiment systems depends on repetition rate of theimpedance measurement. To further reduce power consumption, receiveractivity can be controlled by only performing impedance measurementswhen information exchange is needed by the system in some embodiments.For example, in one embodiment, transmissions are synchronized to atrigger signal or a shift clock signal.

FIG. 1 a illustrates embodiment data transmission system 100 thattransmits data across isolation barrier 108. System 100 has datatransmitter 102 coupled to passive data transmitter 101. In anembodiment of the present invention, passive data transmitter 101 isimplemented by switch 110 coupled to isolation barrier 108 viaelectrodes (not shown). Switch 110 is implemented by a metal-oxidesemiconductor transistor (MOSFET), however, other types of transistorsor devices can be used. Alternatively, passive data transmitter 101 canbe implemented by other circuits that effect a change in impedance. Insome embodiments, a capacitance is modulated by the passive transmitter101. In alternative embodiments, the modulated capacitance can bereplaced by other elements that can be modulated, for example, aresistance. Sensing element 104 has two terminals 170 and 172 coupled toisolation barrier 108. The coupling capacitance between sensing element104 and passive transmitter 101 is represented by capacitances 112 and114.

In an embodiment, data transmitter closes switch 110 when transmitteddata is in a first state, and opens switch 110 when the transmitted datais in a second state. For example, switch 110 is closed or in a lowimpedance state when a logical “1” is being transmitted and open or in ahigh impedance state when a logical “0” is being transmitted.Alternatively, switch 110 is closed or in a low impedance state when alogical “0” is being transmitter and open or in a high impedance statewhen a logical “1” is being transmitted. In alternative embodiments ofthe present invention, multi-level data can be transmitted by furthermodulating the impedance of passive transmitter 101 to includeintermediate impedance states between the high impedance state and lowimpedance state. In embodiments data transmitter 102 can transmit datain an unencoded form, or in a coded form using a coding scheme, such asManchester coding. Alternatively, other coding schemes such asnon-return to zero (NRZ) can be used.

Sensing element 104 detects the impedance between terminals 170 and 172.It can be seen that when switch 110 is closed, the impedance betweenterminals 172 and 170 approximately corresponds to capacitances, 112 and114 in parallel. When switch 110 is open, however, the impedance between170 and 172 is dominated by other parasitic paths and is, therefore,different (higher in this example). During operation of system 100,sensing element 104 measures the impedance between terminals 170 and 172and compares the measured impedance to a threshold. If the measuredimpedance is greater than the threshold, switch 110 is assumed to beclosed and sensing element 104 outputs a first logical state on line105. If, on the other hand, the measured impedance is not greater thanthe threshold, sensing element 104 outputs a second logical state online 105.

Data receiver 106 includes logic for receiving data from sensing element104. In an embodiment, data receiver 106 directly uses the informationfor further data treatment. In other embodiments, the received data isfirst decoded from the data stream delivered by the sensing element,according to the coding scheme (e.g., Manchester Coding) used by thetransmitter.

FIG. 1 b illustrates embodiment data transmission system 120, in whichextra capacitors 122 and 124 are coupled to switch 110 in passivetransmitter 121. Extra capacitance from capacitors 122 and 124 is usedto increase or adjust the change in impedance seen by sensing element104. The sizes of the extra capacitors are chosen to provide detectablechange in impedance seen by sensing element 104. The capacitance valuesof the extra capacitors 122 and 124 are adapted to the resolution of thesensing element 104 to compensate for drift and other parasitic effects.In embodiments where the potential between the transmitter and thereceiver quickly changes (e.g., in power inverters or switched modepower supplies), current is induced into sensing element 104 due to thecapacitive coupling. Extra capacitors 122 and 124 on the transmitterside and the filtering of the measured value on the receiver side areadapted to achieve a sufficient signal to noise ratio to achievedetectable values for data communication.

FIG. 1 c illustrates embodiment data transmission system 130 using adifferential transmission scheme. Data transmitter 102 is coupled todifferential passive transmitter 131 having two switches 142 and 144coupled to the output and input of inverter 148, respectively so thatthe switches 142 and 144 are in opposite states. For example, whenswitch 142 is open, switch 144 is closed, and when switch 142 is closed,switch 144 is open. In an embodiment, switches 142, 144 are coupled toisolation barrier 108 via capacitors 134, 136, 138 and 140. Inalternative embodiments, however, some or all of these capacitors can beomitted depending on the application and its specifications. Thecapacitance of isolation barrier 108 between passive transmitter 131 andsensing element 132 is represented by capacitances 112, 114 and 133.

Sensing element 132, which is coupled to isolation barrier 108 viaterminals 174, 176 and 178, measures a first impedance between terminals174 and 176, and a second impedance between terminals 176 and 178. Ifthe first impedance is greater than the second impedance, sensingelement 132 determines that a first logical state has been transmittedby passive transmitter 131. If, on the other hand, the second impedanceis greater than the first impedance, sensing element 132 determines thata second logic state has been transmitted by passive transmitter 131. Inalternative embodiments, however, a bias is applied to the comparisonbetween the first and second impedance to compensate for asymmetries inthe coupling between the passive transmitter 131 and sensing element132. Using differential transmission and sensing can achieve betternoise immunity achieved in some embodiments.

FIG. 1 d illustrates embodiment transmission system 150 using sensingelement 180 having three terminals 182, 184 and R. Capacitances 154, 152and 156 represent the capacitance of isolation barrier 108 betweenswitch 110 and terminals 182, 184 and R of sensing element 180,respectively. Sensing element 180 measures a first impedance betweenterminals 182 and 184, and a second impedance between terminals 182 andR. If a ratio of the first impedance to the second impedance is greaterthan a threshold, sensing element 180 determines that a first logicstate has been transmitted by passive transmitter 101. If the ratio ofthe first impedance to the second impedance is not greater than thethreshold, sensing element 180 determine that a second logic state hasbeen transmitted by passive transmitter 101.

Alternatively, sensing element 180 determines that a first logic statehas been transmitted by passive transmitter 101 if the first impedanceis greater than the second impedance, and determines that a second logicstate has been transmitted by passive transmitter 101 if the secondimpedance is greater than the first impedance. In a further embodiment,a bias can be applied to this comparison to adjust for asymmetries.Asymmetries can also be adjusted by coupling additional impedance topassive transmitter 101 or sensing element 180. Sensing elements 132 and180 shown in FIGS. 1 c and 1 d can be similar, in some embodiments, suchthat only the modulation strategy of data transmitter 102 is different.In FIG. 1 c, both impedances are modulated (if one impedance isincreased, the other is decreased to achieve a differential effect). InFIG. 1 d, only one impedance is modulated, and the other impedance(reference) is held constant. System 130 of FIG. 1 c requires anadditional switch, compared to FIG. 1 d, however, system 130 of FIG. 1 ccan operate with smaller impedances in some embodiments. For example,smaller impedances can be implemented using smaller capacitances, whichare easy to implement on an integrated circuit.

FIG. 2 a illustrates system 200, in which package 214 having twosemiconductor dies 204 and 206 is mounted on PCB 202. Bondwires 210 and212 couple a passive transmitter on die 204 to a receiver on die 206 viaisolation barrier 208 disposed on die 206. Alternatively, isolationbarrier 208 can be disposed on die 204. The passive transmitter on die204 has a device, for example, a switch that modulates an impedance. Thereceiver on die 206 includes a capacitive sensor. The passivetransmitter and receiver operate according to embodiments describedherein. In an embodiment, isolation barrier 208 is a SiO₂ layer.Alternatively, isolation barrier 208 can be implemented by a nitridepassivation layer, or other insulator.

FIG. 2 b illustrates bi-directional system 220, in which two dies, 224and 226 each have a passive transmitter and a receiver. Passivetransmitter 230 on die 224 is coupled to receiver 232 on die 226 viabondwires 242 and 244 and isolation barrier 236, and passive transmitter234 on die 226 is coupled to receiver 228 on die 224 via bondwires 238and 240 and isolation barrier 236. Isolation barrier 236 is disposed ondie 226. The passive transmitter and receiver operate according toembodiments described herein.

In FIG. 2 c, system 250 is illustrated in which a passive transmitter inpackage 252 is coupled to a receiver in package 254 via an isolationbarrier formed by PCB 202. In package 252, a passive transmitter isdisposed on die 256, which is coupled to package pins 268 and 270 viabondwires 260 and 262, respectively. In package 254, a receiver isdisposed on die 258, which is coupled to package pins 272 and 274 viabondwires 264 and 266, respectively. In alternative embodiments, otherpackage types can be used, for example, packages in which the die iscoupled to the package pins via bump bonded connections. The passivetransmitter in package 252 is coupled to PCB 202 via electrodes 276 and278, and the receiver in package 254 is coupled to PCB 202 viaelectrodes 280 and 282. In embodiments of the present invention,electrodes 276, 278, 280 and 282 are implemented by solder pads,however, in alternative embodiments, other structures can be used toprovide coupling to PCB 202. Packages 252 and 254 are mounted onopposite sides of PCB 202 with sensing electrode pairs positioned acrossfrom each other. Alternatively, package 254 can be mounted on the sameside of PCB 202 as package 252, with pins 272 and 274 coupled to bottomside electrodes 282 and 280 using standard vias. The coupling capacitorsare given by geometrically overlapping conducting areas on the PCB,e.g., an electrode on each side of the PCB. Passive transmitter 296 andreceiver 298 operate according to embodiments described herein.

FIG. 2 d illustrates system 290 in which package 292 has a single die294. Passive transmitter 296 is coupled to receiver 298 via on-chipisolation barrier 291. In an embodiment, on-chip isolation barrier 291is implemented using a trench filled with isolating material between thepassive transmitter 296 and receiver 298. In alternative embodiments,the sensing electrodes are located on both sides of the trench. In otherembodiments, insulating layer 291 can be implemented using other on-chipisolation structures such as silicon on insulator (SOI) or PN junctions.The passive transmitter and receiver operate according to embodimentsdescribed herein.

FIG. 3 a illustrates integrated circuit cross-section 300 that isdivided into two isolated domains, Domain 1 and Domain 2. The twodomains on the substrate are isolated by deep trench 308, to ensure thatthe circuit of Domain 2 maintains operability even if there is a worstcase malfunction in Domain 1, for example, when the power supply ofDomain 1 fails. In an embodiment, deep trench 308 is filled with SiO₂ toprevent current flow between Domain 1 and Domain 2. Such isolation isdesirable in applications such as circuits that control airbagdeployment in automobiles. A passive transmitter 310 in Domain 1 iscoupled to receiver 312 in Domain 2 via internal or external capacitors304 and 306. By using a passive transmitter 310 in the context of anembodiment communication scheme, any energy transferred across theborders of Domain 1 and Domain 2 is limited to a level that does notcause a consecutive fault in an opposite domain.

In an embodiment, the passive transmitter in Domain 1 indicates when thepower supply of Domain 1 allows operation of Domain 1. Here, a permanentindication of the state of Domain 1 to Domain 2 is possible, even ifDomain 1 is in a power down mode.

FIG. 3 b illustrates an embodiment implementation of coupling capacitors304 and 306 (see FIG. 3 a). In an embodiment, coupling capacitors 304and 306 are realized by metal structures from both sides of SiO₂ safetybarrier 330. Capacitor 304 is realized horizontally by metal lines inthe same layer as an interdigitated structure. Alternatively, capacitor306 is implemented vertically by overlapping areas of different layers.In further embodiments, additional layers can be used to increase thecapacitance (e.g., to form a sandwich capacitor) or as shielding layersto avoid interference between adjacent channels, substrate coupling orRF emissions.

FIG. 4 illustrates embodiment capacitive communication system 400suitable for parallel transmission of several data bits including avalidation mechanism by a shift clock signal using synchronous datatransfer. Synchronous data transfer is advantageous in embodiments wherethe transmitter and the receiver do not provide stable clockfrequencies, e.g. due to temperature drift. On the transmission side,system 400 has modulator 404, which is clocked by clock generator 402.Modulator 404 sends a plurality of data streams via switches 424 and 426coupled to synchronous sensor 408 via an isolation barrier representedby capacitances 414, 416, 418 and 420. In alternative embodiments, onechannel or three or more data channels can be transmitted. Furthermore,shift clock 402 operates switch 422 coupled to asynchronous clock sensor406 via the isolation barrier represented by capacitances 410 and 412.

Asynchronous shift clock sensor 406 monitors the state of switch 422 bymeasuring the impedance of the isolation barrier represented bycapacitors 410 and 412. Asynchronous clock sensor 406 then reconstructsthe transmitted shift clock and triggers synchronous sensor 408, toperform impedance measurements of the isolation barrier according to therecovered shift clock. Asynchronous clock sensor 406 and synchronoussensor 408 operate according to embodiment sensing elements describedherein. For example, in one embodiment the modulator changes state withone edge of the shift clock signal and receiver evaluates the impedanceswith the other shift clock edge. As such, the data is transferredsynchronously using the shift clock.

FIGS. 5 and 6 illustrate example embodiments of receiver circuits thatdetect the state of a passive transmitter. Further embodiment receiversand reception schemes are described in co-pending and commonly assignedU.S. patent application: Ser. No. 12/696,932, filed on Jan. 29, 2010,entitled, “System and Method for Testing a Circuit,” which applicationis hereby incorporated herein by reference in its entirety.

FIG. 5 illustrates an embodiment communication system 500 showingpassive transmitter switch 520 coupled to the sensing element ofreceiver IC 502 through an isolation barrier represented by capacitancesCC1 and CC2. The sensing element of receiver IC 502 has receiver 504coupled to a first side of passive transmitter switch 520 viacapacitance CC1, and transmitter 506 coupled to a second side of passivetransmitter switch 520 via capacitor CC2. Impedance ZL1 represents theimpedance between a first side of passive transmitter switch 520 and GNDof the passive transmitter, and impedance ZL2 represents a second sideof passive transmitter switch 520 and GND of the passive transmitter.Impedance ZL3 represents the ground impedances from receiver IC 502 toGND of the passive transmitter.

During operation, in an embodiment, transmitter 506 of the sensingelement transmits an AC signal, which is received by receiver 504 of thesensing element, as base for determining a transmission factor betweentransmitter 506 and receiver 504. The transmission factor comprisesinformation about the attenuation (magnitude) and phase and representsan image of the impedance between transmitter 506 and the receiver 504.If the measured transmission factor (or parts of it) at the receiver isabove a threshold, passive transmission switch 520 is determined to beopen. If the measured transmission factor (or parts of it) is not abovethe threshold, passive transmission switch 520 is determined to beclosed. In one embodiment, transmitter 506 transmits a current andreceiver 504 receives a voltage. Alternatively, transmitter 506 cantransmit either a voltage or current and receiver can receiver either avoltage or current. Therefore, in embodiments, the transmission fromtransmitter 506 to receiver 504 can be a voltage gain, current gain,tranconductance or transresistance. In further embodiments, signal gainsusing mixed currents and voltages can be measured.

According to FIG. 5, there are three return paths for a currentgenerated at transmitter 506:

1. CC2→Switch 620→CC1→Receiver 604;

2. CC2→Switch 620→ZL1→ZL3; and

3. CC2→ZL2→ZL3

When switch 520 is closed and the impedance of path 1 is low compared topath 2 and path 3 in parallel, it can be assumed that a detectable inputsignal can be measured at receiver 504. This condition is fulfilled ifan AC signal is chosen appropriately, mainly by adapting the frequencyand the slew rate of the transmitted signal to avoid unintendedresonance effects with ZLx. Further, the impedance of paths 2 and 3 canbe adjusted by the introduction of the impedance ZL3, if necessary. Whenswitch 520 is open, current along path 1 is due to parasitic coupling(e.g capacitive) between the disconnected parts of passive transmitterswitch 520, whereas paths 2 and 3 always exist. In embodiments, theimpedance in the parasitic return path when the switch is open isdifferent from the impedance in path 1 when the switch is closed,resulting in different transmission factors. Differences in measuredtransmission factors can be directly related to the state of the switchand is usable for monitoring of the switch. In one embodiment, switchstates are determined by comparing the measurement to a threshold. Forexample, if the measured transmission factor (or parts of it) throughpath 1 is above a threshold, passive transmitter switch 520 isdetermined to be closed. If, on the other hand the transmission factor(or parts of it) through path 1 is below the threshold, switch 520 isdetermined to be open.

FIG. 6 illustrates embodiment measurement system 600. The sensingelement of receiver IC 602 has receiver 606 coupled to a first side ofswitch 520 via capacitance CC1, transmitter 608 of the sensing elementcoupled to the first side of passive transmitter switch 520 viacapacitance CC3, and transmitter 604 of the sensing element coupled to asecond side of passive transmitter switch 520 via capacitance CC2.Impedance ZL1 represents the impedance between a first side of passivetransmitter switch 520 and GND, and impedance ZL2 represents theimpedance between a second side of passive transmission switch 520 andGND. Impedance ZL3 represents the ground impedance from IC 602 to GND,respectively.

During operation of receiver IC 602, a first transmission factor (orparts of it) between transmitter 604 and receiver 606 of the sensingelement is measured, then a second AC transmission factor (or parts ofit) between transmitter 608 and receiver 606 of the sensing element ismeasured. If the ratio of the first transmission factor (or parts of it)to the second transmission factor (or parts of it) is greater than athreshold, then passive transmission switch 520 is considered to beclosed. If, on the other hand, the ratio of the first transmissionfactor (or parts of it) to the second transmission factor (or parts ofit) is not greater than the threshold, then passive transmission switch520 is considered to be open. In an alternative embodiment of thepresent invention, receiver 606 is replaced by a single transmitter andtransmitters 604 and 608 are replaced by two receivers. The first andsecond AC transmission factor (or parts of it) can, therefore, besimultaneously measured.

In a further embodiment, electrode pairs can be implemented usingcoreless transformers (CT) as illustrated in the embodiment of FIG. 7.One terminal of the primary winding of coreless transformer CT1 iscoupled to ground via switch S11, and the other terminal of the primarywinding of CT1 is coupled to Vdd via switch S12. In addition, oneterminal of the primary winding of coreless transformer CT2 is coupledto ground via switch S21, and the other terminal of the primary windingof CT2 is coupled to Vdd via switch S22. Secondary terminals E1 and E2from CT1 and CT2, respectively, are coupled to sensor 702. Sensor 702operates according to embodiment sensors described herein. In anembodiment, electrodes E1 and E2 are floating if all switches 511, S12,S21 and S22 are open. If either 511 and S21 or S12 and S22 are closed,however, electrodes E1 and E2 are coupled together.

Because pulses transmitted over the CT are very short, for example, lessthan 20 ns in one embodiment, CT coils are without current most of thetime. Alternatively, other activation times besides 20 ns can be used.In embodiments where the transmitter has power saving mode, datatransfer via the inductive coupling of the CT is disabled during thepower savings mode.

In one embodiment using a CT coil, the CT coil is used to transmitpulses of one polarity. A first connection of a coil is coupled to afirst terminal of the CT transmitter via a first switch and the secondconnection of a coil is directly connected to a second terminal of theCT transmitter. In some embodiments, a second switch is introducedbetween the second connection of the coil and the second terminal of thetransmitter. The coil, therefore, can be separated from the transmitterby opening both switches. If only one of the switches is closed, oneside of the coil is connected to one terminal of the transmitter. Insome embodiments, two independent coils are connected to the transmittervia a similar structure. The modulation of the impedance between bothcoils is achieved by opening all 4 switches to indicate a first stateand by connecting one side of each coil to the same transmitter terminalin a second state. In such an embodiment, each coil can be used as anelectrode for capacitive coupling.

FIG. 8 illustrates an embodiment in which the CT coil is coupled in anH-bridge structure. One terminal of CT coil CT is coupled to Vdd viaswitch S34 and to ground via switch S33. A second terminal of CT coil CTis coupled to Vdd via switch S44 and to ground via switch S43. Sensor802, which operates according to sensor embodiments described herein, iscoupled to secondary terminal E3 and to ground via capacitance Cl atterminal E4. Here, data pulses with both polarities are transferred viathe CT. In an embodiment, electrode E3 is floating if all switches S33,S34, S43 and S44 are open due to bad coupling between of electrodes E3to E4. Electrodes E3 and E4 are coupled if S33 or S43 are closed. Themodulation scheme of the impedance works in a similar way, with the coilbeing one electrode, and one terminal of the transmitter being thesecond electrode.

Advantages of embodiments of the present invention include very lowpower consumption on passive transmitter side and the ability to delivera permanently valid data signal, or slowly changing signals withoutrefresh pulses. Furthermore, in some embodiments, low capacitancesensing electrodes can be used (e.g., below 1 pF for integratedcapacitors on an IC) or a relatively high distance between thetransmitter and the receiver tolerated, for example, up to thecentimeter range to support high isolation capability. Alternatively,the supported ranges can be higher or lower depending on the embodimentat its specifications. The shape of sensing electrodes can be adapted toapplication requirements, and different types of isolation barriers canbe supported, for example on-chip, on PCB, within a package, etc.

Advantages of embodiments further include the ability to place analog ordigital circuit elements under the sensing electrode, for example, inthe case of using a metal layer of a chip for a sensing electrode.

Another advantage of embodiments that include a passive transmitter andan active receiver includes the ability to minimize power consumption onthe transmitter side and expend power on the receiver side where thepower budget is less restricted. This is also advantageous inapplications, such as those using power saving modes, which has aminimal the power budget on the transmitter side.

A further advantage of the capacitive communication channel from asafety point of view is the fact that it is insensitive to “stuck aterrors,” hence only AC signals can pass the capacitive coupling path.Embodiments of the present invention also offer the flexibility to usedifferent modulator and demodulator implementations that are known fordigital RF transmission. However, with respect to embodimentimplementations that are capacitively coupled, the received signals willhave high SNR compared to RF conditions. Therefore, modulators anddemodulators of lower complexity can be used when appropriate.

It will also be readily understood by those skilled in the art thatmaterials and methods may be varied while remaining within the scope ofthe present invention. It is also appreciated that the present inventionprovides many applicable inventive concepts other than the specificcontexts used to illustrate embodiments. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

What is claimed is:
 1. A system for communication comprising: a receiverfor receiving data from a passive transmitter capacitively coupled tothe receiver via a isolation barrier, wherein the receiver comprises asensing element having a first plurality of terminals configured to becapacitively coupled to the passive transmitter and DC isolated from thepassive transmitter; a first terminal for sending a reference signal;and a second terminal for receiving an input signal, wherein a state ofthe passive transmitter is determined based on a transmission factorbetween the reference signal and the received input signal and comparingthe transmission factor or portions of the transmission factor to athreshold.
 2. The system of claim 1, wherein the sensing element isconfigured to receive a differential signal from the passivetransmitter.
 3. The system of claim 2, further comprising the passivetransmitter having a first switch and a second switch, wherein: thefirst switch is ON and the second switch is OFF if a first data state istransmitted; and the first switch is OFF and the second switch is ON ifa second data state is transmitted.
 4. The system of claim 1, whereinthe sensing element measures an impedance between the first plurality ofterminals to determine a transferred data value.
 5. A system forcommunication comprising: a passive transmitter comprising a passivetransmitting element comprising a second plurality of terminalscapacitively coupled to a isolation barrier, the passive transmittermodulating an impedance between the second plurality of terminalsaccording to a transmitted data value; and a receiver for receiving datafrom the passive transmitter capacitively coupled to the receiver viathe isolation barrier, wherein the receiver comprises a sensing elementhaving a first plurality of terminals configured to be capacitivelycoupled to the passive transmitter and DC isolated from the passivetransmitter.
 6. The system of claim 5, wherein the passive transmittingelement comprises a switch coupled between the second plurality ofterminals.
 7. The system of claim 5, further comprising at least oneadditional impedance coupled to at least one of the second plurality ofterminals.
 8. The system of claim 7, wherein the at least one additionalimpedance comprises a capacitor.
 9. The system of claim 5, wherein theisolation barrier is formed by a printed circuit board (PCB).
 10. Thesystem of claim 9, wherein the sensing element is disposed on a firstdevice coupled to the PCB, and the passive transmitter is disposed on asecond device coupled to the PCB.
 11. The system of claim 5, wherein thepassive transmitter and the sensing element are isolated from each otherwithin a package.
 12. The system of claim 5, wherein: the passivetransmitter is disposed on a first portion of an integrated circuit; andthe sensing element is disposed on a second portion of the integratedcircuit.
 13. The system of claim 12, wherein the isolation barriercomprises an insulating layer disposed on an integrated circuit.
 14. Thesystem of claim 13, wherein the insulating layer comprises a nitridelayer.
 15. A semiconductor circuit for receiving data from acapacitively coupled passive transmitter, the circuit comprising: asensing element having a plurality of terminals configured to becapacitively coupled to the passive transmitter via a isolation barrier;and a data receiver coupled to an output of the sensing element, thedata receiver receiving a stream of data from the sensing element,wherein the plurality of terminals of the sensing element comprise afirst terminal and a second terminal, the sensing element measures animpedance between the first terminal and the second terminal, if themeasured impedance exceeds a first threshold, a first data state is sentto the data receiver, and if the measured impedance does not exceed thefirst threshold, a second data state is sent to the data receiver. 16.The semiconductor circuit of claim 15, wherein: the plurality ofterminals of the sensing element comprise a first terminal and a secondterminal; the sensing element measures an impedance between the firstterminal and the second terminal; if the measured impedance exceeds afirst threshold, a first data state is sent to the data receiver; and ifthe measured impedance does not exceed the first threshold, a seconddata state is sent to the data receiver.
 17. The semiconductor circuitof claim 15, further comprising the isolation barrier.
 18. Thesemiconductor circuit of claim 17, wherein the isolation barriercomprises a printed circuit board.
 19. The semiconductor circuit ofclaim 17, wherein the isolation barrier comprises an insulating layerdisposed on an integrated circuit.
 20. A semiconductor circuit forreceiving data from a capacitively coupled passive transmitter, thecircuit comprising: a sensing element having a plurality of terminalsconfigured to be capacitively coupled to the passive transmitter via aisolation barrier; and a data receiver coupled to an output of thesensing element, the data receiver receiving a stream of data from thesensing element, wherein the sensing element comprises a first terminal,a second terminal, and a third terminal, the sensing element measures afirst impedance between the first terminal and the second terminal, thesensing element measures a second impedance between the third terminaland the second terminal, if the first measured impedance is greater thanthe second measured impedance, a first data state is sent to the datareceiver, and if the first measured impedance is not greater than thesecond measured impedance, a second data state is sent to the datareceiver.
 21. A method for receiving data from a passive transmitterthrough a isolation barrier, the method comprising: measuring impedancebetween a plurality of terminals coupled to the isolation barrier; anddetermining a state of the passive transmitter based on the measuredimpedance.
 22. The method of claim 21, wherein measuring the impedancecomprises: measuring a first impedance between a first terminal and asecond terminal; measuring a second impedance between the secondterminal and a third terminal; determining that the passive transmitteris in a first transmission state if the first measured impedance isgreater than the second measured impedance; and determining that thepassive transmitter is in a second transmission state if the firstmeasured impedance is not greater than the second measured impedance.23. The method of claim 21, wherein measuring the impedance comprises:determining a first transmission factor between a first terminal and asecond terminal; determining a second transmission factor between thefirst terminal and a third terminal; determining that the passivetransmitter is in a first state if a ratio of the first transmissionfactor to the second transmission factor is greater than a threshold;and determining that the passive transmitter is in a second state if theratio of the first transmission factor to the second transmission factoris not greater than the threshold.
 24. The method of claim 21, whereinthe isolation barrier comprises a printed circuit board.
 25. A method ofcommunicating across a isolation barrier, the method comprising:transmitting data, wherein the transmitting comprises modulating animpedance coupled to a first portion of the isolation barrier, accordingto the data; and receiving the data, wherein the receiving comprisesmeasuring an impedance of a second portion of the isolation barrieropposite the first portion.
 26. The method of claim 25, whereinmodulating the impedance comprises opening and closing a first switchcoupled to the first portion of the isolation barrier.
 27. The method ofclaim 25, further comprising: transmitting a clock, wherein transmittingthe clock comprises modulating a second impedance coupled to a thirdportion of the isolation barrier, wherein a state of the data isconfigured to transition according to the clock; and receiving theclock, wherein the receiving the clock comprises measuring an impedanceof a fourth portion of the isolation barrier, and the fourth portion isopposite the third portion.
 28. The method of claim 25, wherein theisolation barrier comprises a printed circuit board.
 29. A method forsynchronously transferring data from one side of a capacitive isolationbarrier to another other side of the isolation barrier, the methodcomprising: transmitting a shift clock signal via a first signal pathcoupled to the isolation barrier; changing an impedance between at leasttwo terminals of a second data path coupled to the isolation barrieraccording to data to be transmitted at one edge of the shift clocksignal; and triggering a sensing element to evaluate the impedance at asecond edge of the shift clock signal.
 30. The method of claim 29,wherein the isolation barrier comprises an insulating layer disposed onan integrated circuit.
 31. A semiconductor circuit for receiving datafrom a capacitively coupled passive transmitter, the circuit comprising:a sensing element having a plurality of terminals configured to becoupled to the passive transmitter; a data receiver coupled to an outputof the sensing element, the data receiver receiving a stream of datafrom the sensing element, wherein the sensing element comprises a firstterminal, a second terminal, and a third terminal; the first terminaland the second terminal are configured to be coupled to a first side ofthe passive transmitter; the third terminal is configured to be coupledto a second side of the passive transmitter; the sensing elementmeasures an impedance between the first terminal and the secondterminal; the sensing element measures a second impedance between thefirst terminal and the third terminal; if a ratio of the first impedanceto the second impedance is greater than a threshold, a first data stateis sent to the data receiver; and if the ratio of the first impedance tothe second impedance is not greater than the threshold, a second datastate is sent to the data receiver.